LAUSR

The present study deals with Soret and Dufour effects on the onset of double-diffusive natural convection in a horizontal Brinkman porous layer with a free-stress upper boundary. The thresholds of stationary, oscillatory, and subcritical bifurcations are derived on the basis of linear and non-linear analysis, as functions of the governing parameters. The effect of Dufour ( D u ) and Soret ( S r ) parameters on the thresholds of instabilities is illustrated and discussed. There are critical combinations ( S r , D u ) for which the thresholds of supercritical bifurcations are independent of the buoyancy ratio. Particularly, for D u = 0 and S r = L e / ( 1 + L e ) , the stationary convection starts at the threshold of the pure thermal convection whatever the value of the buoyancy ratio.